The present invention relates to turbomachines having stator vanes that are pivotable about a longitudinal axis of the vane to enable efficient operation of the turbomachine over a range of operating conditions. More particularly, the present invention relates to a support structure for pivotably supporting variable turbomachine stator vanes to reduce the wear between the vane trunnion and the support structure that engages the trunnion.
Modern day turbomachines incorporate variable geometry elements to improve the operating efficiency of the machine at conditions that are different from the design point applicable to a fixed geometry machine. For example, gas turbine engines having axial-flow compressors incorporate variable geometry elements such as variable inlet guide vanes and variable stator vanes. The variable vanes have trunnions at their upper and lower extremities and are arranged to be pivotable about a longitudinal axis, to enable their angle of attack relative to an incoming air stream to be changed in accordance with a predetermined schedule. Because of repetitive pivotal movements of the variable stator vanes over time, wear can occur at the contact areas between the vane trunnions and the stationary support structure that supports the vane trunnions for pivotal movement. Oftentimes the vane trunnions are carried in metallic bushings for reduced wear.
In aircraft gas turbine engines, where weight considerations are very important, in many instances lighter weight aluminum alloy materials are utilized for variable stator vane support structures, where appropriate, to minimize weight. In some engines, the variable stator vanes are supported in aluminum alloy support rings, which generally have a higher coefficient of thermal expansion than do the harder materials from which the vanes and the vane trunnions are formed, as well as those metallic materials sometimes utilized in vane trunnion bushings.
When a gas turbine engine warms up from a cold start to normal operating temperature, the temperature rise within the compressor can be as much as about 700xc2x0 F., depending upon the compressor stage at which the temperature change is measured. As a result, the greater thermal expansion of the aluminum alloy trunnion support structure results in a radial gap between the trunnion support bushing and the surrounding structure, which allows the trunnion support bushing, which can have a lower coefficient of thermal expansion, to pivot relative to the trunnion support structure. Such relative movement can result in wear of the softer aluminum alloy support structure material and, if the wear is sufficiently large, it can allow movement of the stator vane into the annular space swept by an adjacent upstream or downstream rotor, resulting in contact between the stator vane and the rotor or rotor blades and possible damage to either or both the rotor blades or the stator vanes.
Sometimes composite materials are utilized for the trunnion bushing support structures in an effort to provide a material that has a lower coefficient of friction. Although composite materials, such as polyimide-based synthetics, can be utilized, their wear characteristics may be such that more frequent replacement of such composite bushings is required.
The present invention is directed to minimizing wear between the trunnion bushing and the trunnion support structure.
Briefly stated, in accordance with one aspect of the present invention, a stator vane support structure is provided for supporting a turbomachine stator vane for pivotal movement about a stator vane pivot axis. The support structure includes a carrier member for supporting a trunnion that extends from a longitudinal end of a stator vane. The carrier member is formed from a material having a first coefficient of thermal expansion, and it includes openings for receiving stator vane trunnion bushings. A substantially cylindrical bushing is carried in the opening in the carrier member and is formed from a material that has a second coefficient of thermal expansion that is less than the first coefficient of thermal expansion. The bushing includes a tubular body wall within which the trunnion is received for relative rotation therewith. The tubular body wall includes a gap to allow the outer diameter of the bushing to be decreased by the application of a laterally applied compressive force. The tubular body will in a relaxed condition have an initial outer diameter that is larger than the diameter of the opening in the carrier member. Thus, upon lateral compression of the bushing to a smaller outer diameter than its initial outer diameter it can be inserted into the opening in the carrier member and can be retained therein by an interference fit upon release of the compressive force.